Sign for a line (3)
I believe the answer is:
cue
'sign for a line' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for cue that I've seen before include "Billiards stick" , "Prompt; reminder" , "Tapering stick used by snooker player" , "strike" , "Sporting rod" .)